Method for building and transmitting a watermarked content, and method for detecting a watermark of said content

ABSTRACT

The present invention concerns a method for building a watermarked content for sending to at least one user unit having a user unit identifier, the watermarked content comprising a first series of packets, at least some from the first series of packets being available in at least two different qualities, wherein said method comprises the steps of:
         generating a watermark based on at least the user unit identifier, the watermark being formed by a first series of values from a predetermined set of values;   mapping each of the at least two different qualities to one from the predetermined set of values;   building the watermarked content by sequentially selecting from the first series of packets, a packet having a quality corresponding to each value from the first series of values forming the watermark.       

     It further concerns a method for transmitting such a watermarked content

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/948,527 filed Jul. 23, 2013, which claims priority under 35 U.S.C.Section. 119 to the U.S. provisional Application No. 61/674,869 and tothe European patent application No. EP12177576.1, entitled “METHOD FORMARKING AND TRANSMITTING A CONTENT AND METHOD FOR DETECTING ANIDENTIFIER OF SAID CONTENT” filed Jul. 24, 2012, the contents of whichare hereby incorporated by reference herein.

TECHNICAL FIELD

The present invention concerns a method for building a watermarkedcontent for sending to at least one user unit having a user unitidentifier, the watermarked content comprising a first series ofpackets, at least some from the first series of packets being availablein at least two different qualities.

The invention further concerns a method for transmitting a watermarkedcontent from a content provider to at least one user unit having a userunit identifier, the watermarked content comprising a first series ofpackets, at least some from the first series of packets being availablein at least two different qualities,

The method also concerns a method for detecting a watermark of a contentbuilt or transmitted according to the method of the invention.

BACKGROUND ART

Presently, when an audio/video stream or other content such as pay-TVcontent or similar are delivered to a user by a provider, the contentprovider may be willing to watermark this content in a way that isunique for each user. Thus, if the content has been illegallyredistributed, the provider may find the source of this illegaldistribution.

Usually, a watermark is applied to the content itself, the watermarkbeing formed by an invisible/inaudible modification of the video and/oraudio stream. For unicast streams, it is possible to apply the watermarkat the head-end or at the reception device. At the head-end, applying awatermark requires modifying the stream for each user. This solutionrequires a lot of resources and is quite difficult to implement.

If the watermark is applied at the reception, the user units must besecure devices, which is usually not the case if the user unit is acomputer type receiver. Moreover, applying the watermark at thereception is a heavy processing.

Thus, there is a need for a method enabling applying a watermark to acontent, which is compatible with a unicast stream.

The publication of P. Y. Liew and M. A. Armand, entitled “Inaudiblewatermarking via phase manipulation of random frequencies”, “MULTIMEDIATOOLS APPL.” vol. 35, no 3, 6 Jun. 2007 (2007-06-06), XP019555269, pages357-377 describes a method for watermarking an audio content in a robustand inaudible way. This method exploits the fact that the human auditorysystem is not sensitive to absolute phase of a signal. Thus, the humanauditory system is unable to discern any audible difference between twosignals of the same frequency, but for example 60° out of phase whenboth are heard separately.

The method described in this document comprises the following steps.

1. Generate, via a pseudorandom generator, 300 random numbers between 0and 0.5.

2. Multiply each number generated by 44.1×10³ to yield a frequency valuebetween 0 and 22.05 kHz.

3. Sort the corresponding 300 frequency components of the audio segmentin terms of their amplitudes.

4. Identify 200 frequency values corresponding to the frequencycomponents of smallest amplitude. Then, without changing the order inwhich they were generated, partition them into two groups of equalcardinality with the first 100 frequency values contained in the firstgroup.

5. To insert watermark bit 0, the phase values of the frequencycomponents of the audio segment corresponding to the first and secondgroups are set to −2π/3 and 2π/3 rad, respectively. To insert watermarkbit 1, the phase values of the frequency components of the audio segmentcorresponding to the first and second groups have their phase values setto 2π/3 and −2π/3 rad, respectively.

This method differs from the method of the present invention at least bythe fact that the method of the present invention does not rely onseveral signal being out of phase.

The document above contains a short description of several methodsenabling adding a watermark to an audio content. According to one ofthose methods, referred to as “Time base modulation”, the audio signalis first subdivided into segments. Each segment is then expanded orcompressed depending on the value of a watermark bit for this segment.

To retrieve the watermark, the watermarked sequence is compared to theoriginal to detect the deviations resulting from the various sequencesof expansions and compressions.

This method requires the comparison between the original signal and thewatermarked signal in order to retrieve the watermark as, whatever thewatermark is, the quantity of information within one packet remainsunchanged. Moreover, the watermark may be audible if the expansions orcompressions changes are too great.

The publication entitled “Length based network steganography using UDPprotocol” from Anand S Nair, Abhishek Kumar, Arijit Sur and SukumarNandi, 3RD INTERNATIONAL CONFERENCE ON IEEE, 27 May 2011, pages 726-730,XP032050871, ISBN: 978-1-61284-485-5 discloses a steganographicalgorithm for hiding secret data within packets of content. The lengthof the packets is modulated in order to correspond to the secret data tobe transmitted. The algorithm described in this document enablesimitating the length distribution of conventional packets, i.e. packetswithout steganographic data, in order is to prevent users from guessingthat secret data is added to the data flow.

In this document, the key is the use of steganography. This means thatdata is hidden within the packets. If an ill-intentioned person learnsthat secret data is hidden within the data, this data can be easilyeliminated and even possibly read. Moreover, the secret data is verysensitive to re-sampling.

DISCLOSURE OF THE INVENTION

The object of the invention is to solve the problem of providing amethod enabling to watermark a content in a way which is unique for auser unit or at least for a group of user units. This watermarking isnot detected by the user and does not modify the quality or the speed oftransmission of the transmitted data. Moreover, even if anill-intentioned user detects or knows that a watermark is embedded inthe transmitted data, he/she can only remove this watermark at the costof a loss of quality. In case a content is found for example on anunauthorized site, the source of the content can be tracked by readingthe watermark.

The object of the invention is achieved by a method for watermarking acontent such as defined in the preamble and characterized in that saidmethod comprises the steps of:

-   -   generating a watermark based on at least the user unit        identifier, the watermark being formed by a first series of        values from a predetermined set of values;    -   mapping each of the at least two different qualities to one from        the predetermined set of values;    -   building the watermarked content by sequentially selecting from        the first series of packets, a packet having a quality        corresponding to each value from the first series of values        forming the watermark.

The object of the invention is further achieved by a method fortransmitting a watermarked content such as defined in the preamble andcharacterized in that said method comprises the steps of:

-   -   generating a watermark based on at least on the user unit        identifier, the watermark being formed by a first series of        values from a predetermined set of values;    -   mapping each of the at least two different qualities to one from        the predetermined set of values;    -   building the watermarked content by sequentially selecting from        the first series of packets, a packet having a quality        corresponding to each value from the first series of values        forming the watermark;    -   transmitting said packets to said at least one user unit.

The object of the invention is also achieved by a method for detecting awatermark of a content watermarked according to the method describedabove, characterized in that it comprises the steps of:

-   -   determining a quality of packets forming said watermarked        content;    -   associating a value to the packets of the watermarked content,        said value being determined according to a predetermined rule;        and    -   calculating the watermark from the series of values associated        to the packets forming the content.

According to the invention, the method enables defining an identifierthat may be unique for each user unit or for a given set of user units,for example for all the user units of the same user. This identifier isdefined by using chunks or packets of data having specificcharacteristics, and by mapping the specific characteristics with avalue corresponding to the identifier. These characteristics aredetectable even if the packets are resent from one user unit to another.The characteristic used in the method of the invention is the quality ofthe packets, or in other words, the bit rate at which the data have beenencoded.

The different qualities correspond to different values. The watermark isformed of a set of values which is specific for each user unit or groupof user units. Thus, if a content is analyzed, it is possible todetermine the quality of each packet. The quality of each packet isassociated with a value according to a set of rules, a set of analyzedpackets corresponding to a set of values. The set of values gives a codeor watermark which can be associated with a specific user unit or aspecific user.

According to this method, there is no need for comparing the watermarkedsignal with the original signal to retrieve the watermark. The watermarkcontains by itself, the information that is requested to form thiswatermark. The quantity of information within one packet depends on thevalue of the watermark. This quantity of information can be measured andcan be used to determine the value of the watermark without requiring acomparison with the original signal.

BRIEF DESCRIPTION OF DRAWINGS

The present invention and its advantages will be better understoodthanks to the enclosed drawings and to the detailed description ofdifferent embodiments, in which

FIG. 1 a is a schematic representation of values associated with packetsof data with different qualities;

FIG. 1 b shows a watermark transmitted according to the method of theinvention and with the values of FIG. 1 a;

FIG. 2 a represents a second embodiment of values associated withpackets having different qualities;

FIG. 2 b shows a watermark sent according to the embodiment of FIG. 2 a;

FIG. 3 a is a schematic illustration of a third embodiment of the methodof the invention;

FIG. 3 b illustrates a watermark transmitted according to the embodimentof FIG. 3 a;

FIG. 4 a represents a further embodiment of the method of the invention;

FIG. 4 b shows a watermark sent according to the embodiment illustratedby FIG. 4 a;

FIG. 5 illustrates a concrete implementation of the method of theinvention, according to a particular embodiment;

FIG. 6 a illustrates a first value associated with a set of packets,according to a variant of the method of the invention;

FIG. 6 b illustrates a second value associated with a set of packets,according to the variant of FIG. 6 a; and

FIG. 6 c illustrates a set of packets corresponding to a watermark,according to the embodiment of FIGS. 6 a and 6 b.

MODES FOR CARRYING OUT THE INVENTION

As it is well known from the man skilled in the art, prior to sendingaudio/video data to user units, the content is separated in packets orchunks and processed to lower the size of the data to be transmitted.Usually, this processing or encoding used in the field of the pay-TV forlowering the size of the data are lossy data compression algorithms.This means that some loss of information occurs. There is a trade-offbetween information loss and the reduction in size. The information losscan be used as a definition of the quality or resolution of a content orof a packet. A high resolution or high quality data packet means thatthe loss of information is low and that the packet has a large size.Conversely, a low resolution or low quality data packet has a smallersize, but the loss of information is greater. The compression rate canalso be used to define the quality or resolution of a packet. A high iscompression rate leads to a small sized packet and to a low quality. Alower compression rate leads to a packet having a larger size and abetter quality or resolution.

The bit rate or bandwidth is the quantity of information or number ofbits that are transmitted per unit of time. With a high bit rate or ahigh bandwidth, a great number of packets can be transmitted per unit oftime, or packets having a great size can be transmitted per unit oftime. Consequently, high quality content can be transmitted in case ofgreat bandwidth, while lower quality content can be transmitted in caseof lower bandwidth.

It should be noted that the quality of a transmitted packet remainsunchanged at the receiver reception's side, which means that thisquality can be detected even after the content is redistributed.

In FIG. 1 a, two data packets are represented. One of these packets,illustrated on the left hand side, corresponds to a first quality and isassociated with a first value, for example 0. The other packetcorresponds to another quality and is associated with another value, forexample 1.

FIG. 1 b illustrates an identifier or watermark that is associated witha content and sent according to the method of the invention. The firstset of 8 values corresponds to a synchronisation pattern. Thissynchronisation pattern is useful to detect the beginning of thetransmission of packets from which the watermark may be detected. In theexamples illustrated, the synchronisation pattern comprises four zeros,followed by four ones. After the synchronisation pattern, the qualitiesof subsequent packets correspond to the values forming the watermark. Inthe examples of FIGS. 1 b, 2 b and 3 b, the watermark is {01100010}. Ofcourse, the number of bits of this watermark determines the maximalnumber of codes that it is possible to build. In practice, the number ofbits will usually be greater than eight. It should be noted that thedetection of the packets corresponding to the watermark could be madenot by a synchronisation pattern but for example by a header containedin a packet. In the case of a header, when the packets are received by auser unit, the content of the packets is read. One packet can contain aspecific header implying that this packet is considered as indicatingthe beginning of the watermark. The synchronisation pattern or theheader could be sent immediately before the transmission of the packetcorresponding to the watermark. However, it is also possible to send thewatermark before the synchronisation pattern or to leave a space betweenthe synchronisation pattern and the beginning of the watermark.

According to a particular embodiment, the packets are numbered. This maybe useful if the packets are not displayed immediately by the receiveras soon as they are received, but are rather stored before beingdisplayed. In this case, the packets can be received in non consecutiveorder, i.e. in an order which does not correspond to packets that areconsecutive when displayed. In this case, there is no need for asynchronization pattern as the number of the packets plays the role of aheader. In such a case, the watermark is formed of a succession ofvalues associated with packets identified by specific numbers.

It should be noted that the watermark can be formed by non-consecutivepackets or even by packets in any order. For example, the watermark canbe formed by the value associated with the packet number 8, followed bythe value associated with packet number 12, followed by the valueassociated with packet number 3 and so on.

In this embodiment, it is possible to retrieve the watermark even if thepackets are sent, stored or displayed in an order which is not thenormal forward order. This also enables retrieving the watermark in caseof trick modes for example.

In the embodiment of FIG. 1 b, each packet is associated with the value0 or 1. The management center or the data provider stores data packetsencrypted or encoded according to each quality. Thus, the providerstores a first packet with a first quality and the same packet with asecond quality.

When a watermark or a pattern begins with a first value equal to zero,or in other words, when a first value, for example 0, must betransmitted to a given user unit, the provider sends the packet having aquality corresponding to the value 0, i.e. the packet having the firstquality. If, as in the examples, the second value of the watermark is 1,the provider sends a second packet, corresponding to the following setof data, with the second quality, i.e. the quality corresponding to thevalue 1. For each value of the watermark, the provider sends the packethaving the corresponding quality. The mapping between the quality andthe corresponding value follows a rule that can be chosen depending onthe specific implementation of the method of the invention, but whichmust be known for watermarking a content and for detecting a watermarkfrom a content. Thanks to this rule, it is not necessary to compare theoriginal content with the watermarked content for determining thewatermark.

It should be noted that, in conventional systems, the content is usuallynot stored only once in a single server, but several times on a contentdelivery network (CDN). The content could be stored in the form ofpackets, having possibly several qualities for a same content. Thiscould in particular be done for adaptive streaming. In these systems,the user unit requests a specific packet to the content provider or thecontent delivery network. The CDN or the provider sends the packet tothe concerned user unit. The provider or the CDN have no capacities forchoosing between several packets; they only have the capacity of sendinga packet corresponding to the request. In the present invention, twodifferent embodiments may be used. According to a first embodiment andcontrary to the conventional systems described above, the provider orCDN has means for selecting a given packet among a set of packets.

When a specific user unit requires a given packet, this user unit sendsa corresponding request to the selection means, together withcredentials and identification means. The selection means verifies thecredentials and determines from the identification means, which valuesshould be associated with the packets to be sent. The selection meansforms a request corresponding to the requested values and transmits therequest to the provider or CDN. The provider then replies by sending thecorresponding packet.

According to a second embodiment, illustrated by FIG. 5, the selectionis not made by the provider or CDN, but by an authentication server.Thus, a conventional provider or CDN can be used.

According to this embodiment, the user unit first sends a request to theauthentication server AS which may be independent from the provider.

The request contains means for authenticating the user unit. If theauthentication succeeds, the authentication server determines the userunit identifier and generates a token corresponding to said user unitidentifier. This token is sent to the concerned user unit, usually inencrypted form.

Once the user unit has the token, it is sent to the provider. Thisprovider checks if the token is valid. If this is the case, the providersends the packets to the user unit. The content of the token defineswhich packets must be sent to the user unit. As the token depends on theuser unit identifier, the packets sent to the user unit also depend onthis identifier.

In this embodiment, the provider does not require means for selectingthe packets to be sent. The provider simply sends the requested packetsaccording to the value of the token.

In the embodiment of FIG. 2 b, one value can be associated with morethan one quality, i.e. for example two different qualities. The value 0can be associated with a first quality, corresponding for example to abit rate of 480 kb/s and with a second quality, corresponding forexample to a bit rate of 680 kb/s. The value 1 can be associated with afirst quality, for example 500 kb/s and with a second quality, forexample 700 kb/s. In a preferred embodiment, the high quality associatedwith the value 0 is higher than the low quality associated with thevalue 1.

According to this embodiment, it is possible to take into account, thebandwidth and CPU capacities of the user units to which the packets mustbe sent. For a user unit having always a high capacity bandwidth, it ispossible to use the packets having the best quality or resolution.Conversely, for a user unit having a low bandwidth capacity, theidentifier can be sent by using the poor or lower quality packets.According to this embodiment, it is also possible to choose the type ofpackets to be sent dynamically, by taking into account the availablebandwidth at a given time. This corresponds to adaptive streaming.

In this case, as it can be seen from FIG. 2 b, the same value does notalways use packets having the same quality since one value can be sentby using one of two different packet qualities. This also applies to thesynchronisation pattern.

As in the embodiment described with reference to FIG. 1 b, along withthe synchronisation pattern, the watermark can be sent once per content,several times per content or all the packets could be used to indicatevalues forming the watermark. In the cases where not all packets areused to convey the watermark, conventional adaptive streaming can beused for these packets conveying no watermark. In the case that thewatermark is sent during the whole event or a substantial part of theevent, a “sort” of adaptive streaming can also be used.

If the available bandwidth is high, the high quality packet can be usedto represent the values of the watermark. Conversely, if the availablebandwidth is low, the low quality packet can be used to represent thevalues of the watermark.

In the embodiment illustrated by FIGS. 3 a and 3 b, each value is notassociated with one packet, but with two packets. More specifically, adata packet can have one of the two available qualities, for example lowquality or high quality. Two consecutive packets must be used to definea value. In the example of FIG. 3, the value 0 is associated with twoconsecutive low quality packets or with a high quality packet followedby a low quality packet. The value 1 is associated with either a lowquality packet followed by a high quality packet or with two consecutivehigh quality packets. It should be noted that the choice of associatingone value with a kind of packets is completely arbitrary. It should alsobe noted that one value could be given not by two consecutive packets asin the example of FIG. 3, but by three or more packets.

The left hand part of FIG. 3 b illustrates the synchronisation pattern{0;0;0;0;1;1;1;1}. As it can be seen from this Figure, the same value(for example 0) can be sent in two different ways; either twoconsecutive low quality packets or a high quality packet followed by alow quality packet. The same applies for sending the value 1. The choiceof using one combination of packets rather than the other available onecan be arbitrary or random. It can also take into account, the availablebandwidth. If the available bandwidth is important, zeros can betransmitted by using the type containing a high quality packet. On thecontrary, if the available bandwidth is low, zeros would be sent byusing only low quality packets. For transmitting the value 1, in case ofhigh bandwidth, the type containing only high quality packets can beused. In case of low bandwidth, the value 1 can be sent by using one lowquality packet followed by a high quality packet.

As previously, the identifier can be repeated cyclically or spread amongthe data packets. In case the watermark does not fill the whole content,conventional adaptive streaming can be used. Partial adaptive streamingcan be used even if the watermark is sent during the whole content.

FIG. 4 concerns an embodiment in which the watermark is not formed oftwo values, but three, for example −1, 0 and +1. Moreover, in thisexample, each value corresponds to two different qualities. Thus, atleast a part of the packets used to send an identifier are encodedaccording to six different qualities corresponding to low or highquality for each of the values −1, 0 and +1.

In FIG. 4 b, the left part shows the synchronisation pattern{0;0;0;0;1;1;1;1}. Zeros, minus one and plus one can be sent accordingto two different qualities. The qualities for a given packet can berandom, can be chosen arbitrarily or can depend on the availablebandwidth.

FIGS. 6 a, 6 b and 6 c concern an embodiment which is particularlyadapted to the case where it may be difficult to guarantee a goodsynchronization between the signal to be detected and the sampling.Desynchronization can lead to misdetection of the watermark, suchdesynchronization being likely to arise when a long series of values aresent without intermediate resynchronization.

In order to avoid such desynchronization, this embodiment uses the factthat the packets are available in two different qualities. As in theother embodiments, a specific succession of packets is associated withthe value 0 and another succession of packets is associated with thevalue 1.

In this embodiment, the pattern used to detect an element of thewatermark is not a static element, but derives from the dynamic behaviorof the packets.

More specifically, the succession of a high quality packet followed by alow quality packet opens a time window. The succession of qualities ofthe packets during this time window corresponds to the value that issent. For example, if during the time window, a succession of a highquality packets, followed by a low quality packet arises, the associatedvalue is considered as being 0. On the contrary, if there is no “drop”of quality during the time window, the associated value is 1.

FIG. 6 a illustrates an example of a group of packets comprising a startpattern, followed by other packets demonstrating a drop of qualityduring the time window. This feature corresponds to the value 0.

FIG. 6 b is similar to FIG. 6 a except that there is no drop of qualityduring the time window. This feature corresponds to the value 1.

FIG. 6 c illustrates the transmission of a watermark having the value0110. This embodiment is self-synchronized at the beginning of eachvalue transmission period. This avoids cumulative jitter which may arisewhen a long series of values must be sent without intermediatesynchronization.

When a content is received or detected and when a watermark must beretrieved from this content, the quality of each packet is determined.This determination may be done by reading a relevant piece ofinformation which is contained within the packet. According to anotherembodiment, the determination of the quality can also be done byanalyzing the content of the packet itself and by calculating thequality according to the result of this analysis.

In a first example, it is assumed that the watermark has been setaccording to the embodiment of FIG. 1 b. According to this embodiment,the quality of each packet is determined. If this quality is low, i.e.it corresponds for example to a high compression rate, the value isconsidered as being 0. If the quality corresponds to a high quality,i.e. a low compression rate, the value associated to this packet is 1.This determination is made for each packet. In order to detect thewatermark, the series of values is determined. If a given series ofvalues correspond to a synchronization pattern, the watermark is formedby the values indicated by a given rule, known from the entity in chargeof the watermarking and the entity in charge of retrieving thewatermarking. An example of such a rule is using the n packets followingthe synchronization pattern to form the watermark.

If the watermark uses the embodiment of FIG. 2, the determination of thequality of a packet is made as for the embodiment of FIG. 1. However, inthis case, four qualities can be found. Two qualities correspond to thevalue 0 and two other qualities correspond to the value 1.

In the embodiment of FIG. 3, the quality of the packets is alsodetermined. However, one value is obtained by using two packets.

In the embodiment of FIG. 6, the quality of each packet is determined.If a drop of quality, i.e. the transition from a high quality packet toa low quality packet is detected, a time window opens. In this context,a drop of quality means the transition from a quality above a giventhreshold to a quality below a threshold, which can be the same oranother one.

The quality of the packets during this time window is measured. If thereis a drop of quality during this time window, the value associated tothis group of packets is considered as being 1. If there is no drop ofquality during this time window, the value of the group of packets isconsidered as being 0. In this case, the synchronization patterncorresponds to a quality drop. If there is no quality drop outside ofthe time window, the qualities of the packets are not used fordetermining the watermark.

Several combinations of the different embodiments as well as severaladaptations of these embodiments are possible. For example, it ispossible to use three or more values to define a watermark. The use ofthese three or more values can be made in the case where one value isassociated with one quality or in case one value is associated withseveral qualities.

It is possible to have one value, for example 0, associated with two ormore qualities and to have another value, for example 1, associated toanother number of qualities, for example only one.

In the embodiment of FIG. 3 a or b, two packets are used two define onevalue. It is possible to use more than two packets to define one value.Furthermore, in the embodiment of FIG. 3, the packets can correspond totwo different resolutions or qualities, for example low and highquality. It is possible to define more than two resolutions or qualitiesfor each packet.

If different possibilities are available for sending the same value, thechoice of the used possibility may be arbitrary, may be decided randomlyor may depend on the available bandwidth.

The watermark may be sent during the whole transmission of the content.It could be sent several times during the transmission of the content,without these “several times” covering the whole content, or it can besent only once. If the watermark is not sent during the whole content,it can be sent at given times during the transmission or randomly, inorder to prevent a user from being able to determine when the watermarkis sent or if a watermark is sent.

When the watermark is not sent, the transmission of the packets can bemade always with the same quality, or if available, with differentqualities. In this case, the resolution or quality can be random orarbitrary or depending on the available bandwidth.

At the provider's side, if a packet is used to transmit a watermark,this packet must be encoded according to the watermark to betransmitted. This means that at least two versions of the same packetmust be available by the provider. For the packets that are not used totransmit a watermark, it is possible to have only one version of eachpacket.

However, it is also possible to have several versions. This can beinteresting for using the adaptive streaming as well as for hiding thewatermark within the content. An ill-intentioned person will havedifficulties in determining that a watermark is hidden among the packetsand it will also be difficult to determine where it is hidden.

It is also possible to determine a watermark not by consecutive packets,but by packets at given locations within the content. Thus, thewatermark will be even more difficult to detect by a non authorizedperson.

Thus, this invention provides an effective method for watermarking acontent for unicast transmission.

1. A user unit for detecting a watermark of a content comprising a firstseries of packets, at least some from the first series of packets beingavailable in at least two different resolutions or compression rates,said user unit being configured for: determining a resolution orcompression rate of packets forming said watermarked content;associating a value to the packets of the watermarked content, saidvalue being determined according to a predetermined rule; andcalculating the watermark from the series of values associated to thepackets forming the content.
 2. The user unit according to claim 1,wherein the different resolutions or compression rates available for apacket are associated with at least two different values from saidpredetermined set of values.
 3. The user unit according to claim 2,wherein each different resolution or compression rate of a packet isassociated with a different value.
 4. The user unit according to claim2, wherein several different resolutions or compression rates availablefor a packet are associated with a same value.
 5. The user unitaccording to claim 2, wherein a value from the series of values formingthe watermark is associated with the resolutions or compression rates ofa single packet.
 6. The user unit according to claim 2, wherein a valuefrom the series of values forming the watermark is associated with theresolution or compression rate of at least two consecutive packets. 7.The user unit according to claim 1, wherein the watermarked contentfurther comprises a synchronization pattern, the synchronization patterncomprising a second series of values associated with a second series ofpackets.
 8. The user unit according to claim 1, wherein the packetscomprise a packet identification number.
 9. The user unit according toclaim 1, wherein the resolution or compression rate of a packettransmitted to the user unit and corresponding to a value of thewatermark further depends on a bandwidth available between the contentprovider and said user unit.
 10. A server computer for building awatermark for content to be sent to at least one user unit having a userunit identifier, the content comprising a first series of packets fromwhich at least some packets are available in at least two differentresolutions or compression rates, said server computer being configuredfor: generating the watermark based on at least the user unitidentifier, the watermark being formed by a first series of values froma predetermined set of values; mapping each of the at least twodifferent resolutions or compression rates to one of the values from thepredetermined set of values; and sequentially selecting, from the firstseries of packets available in at least two different resolutions orcompression rates, a packet having a resolution or compression ratemapped to each value from the first series of values forming thewatermark.
 11. The server computer according to claim 10, wherein thedifferent resolutions or compression rates available for a packet areassociated with at least two different values from said predeterminedset of values.
 12. The server computer according to claim 11, whereineach different resolution or compression rate of a packet is associatedwith a different value.
 13. The server computer according to claim 11,wherein several different resolutions or compression rates available fora packet are associated with a same value.
 14. The server computeraccording to claim 11, wherein a value from the series of values formingthe watermark is associated with the resolutions or compression rates ofa single packet.
 15. The server computer according to claim 11, whereina value from the series of values forming the watermark is associatedwith the resolution or compression rate of at least two consecutivepackets.
 16. The server computer according to claim 10, wherein thewatermarked content further comprises a synchronization pattern, thesynchronization pattern comprising a second series of values associatedwith a second series of packets.
 17. The server computer according toclaim 10, wherein the packets comprise a packet identification number.18. The server computer according to claim 10, wherein the resolution orcompression rate of a packet transmitted to the user unit andcorresponding to a value of the watermark further depends on a bandwidthavailable between the content provider and said user unit.
 19. Theserver computer according to claim 10, wherein said server computer:determines the bandwidth available between the content provider and saiduser unit; determines a subset of packets according to the availablebandwidth and the values forming the watermark to be transmitted; andchooses among the packets of said subset of packets, at least one packethaving a resolution or compression rate corresponding to the value to betransmitted among the series of values forming the watermark.
 20. Theserver computer according to claim 10, wherein said server computerdetermines at least one resolution or compression rate of the packetscorresponding to said watermark and sends a request to the contentprovider requiring the transmission of the corresponding packet to theconcerned user unit.